Two cars of the same mass run head-on into each other at $C$. After the collision, the cars skid with their brakes locked and come to a stop in the positions shown in the lower part of the figure. Knowing that the speed of car $A$ just before impact was $5 \mathrm{mi} / \mathrm{h}$ and that the coefficient of kinetic friction between the pavement and the tires of both cars is $0.30,$ determine $(a)$ the speed of car $B$ just before impact, $(b)$ the effective coefficient of restitution between the two cars.